The present invention relates generally to processing signals for storage on a data storage medium such as an optical disc. More specifically, DC control for a multilevel signal is disclosed.
Commercial optical data storage systems, such as CD-ROM and DVD-ROM, use binary recording levels to store information. In general, reading data stored on an optical disc may result in a binary read signal containing significant DC and low frequency components. In this specification, DC and low frequency components will be collectively referred to as low frequency components and DC control will be used to refer to the control of low frequency components, as well as DC components. Such low frequency components interfere with servo-motor controls that are used to position the read head in the optical drive. Therefore, some type of line coding is provided in most systems to control the DC and low-frequency content of the binary signal that is written to and read from the disc.
In CD-ROM and related families (CD-R, CD-RW, etc.), user data is encoded with Eight to Fourteen Modulation (EFM) code to provide run-length limits (RLL) and DC control (DCC). FIG. 1A is a block diagram illustrating an encoding system using EFM. An EFM encoder 102 encodes each byte (eight bits) of user data 100 as a codeword 104 consisting of fourteen channel bits. A bit merger 106 inserts three xe2x80x9cmergexe2x80x9d or xe2x80x9cgluexe2x80x9d channel bits 110 between adjacent codewords to maintain RLL constraints across pairs of codewords and to provide DCC. A differential modulator 108 differentially encodes the channel bits into binary levels. The binary levels are represented as lands and pits on the surface of the optical disc by interpreting xe2x80x9conexe2x80x9d channel bits as transitions from land to pit or vice versa, and xe2x80x9czeroxe2x80x9d channel bits as no transition.
If the number of lands to be written is significantly larger than the number of pits, or vice versa, the DC content of the written signal becomes significant. The bit merger prevents that from happening by introducing a transition (i.e., a channel xe2x80x9conexe2x80x9d merge bit), when necessary, to flip the following codeword, thus changing lands in that codeword to pits and vice versa. DCC encoder 112, which is shown encompassing the bit merger and differential modulator, determines whether or not to introduce such a transition. FIG. 1B is a diagram illustrating a data bit stream that is encoded using EFM and stored on a CD. Each 8 bit data byte 120 is converted to a 14 bit codeword 122. Merge bits 124 are added between codewords 122 to introduce transitions where necessary to balance out the number of pits and lands written to the disc. The binary signal written to the disc 126 transitions whenever a 1 is encountered in the channel code, which includes the 14 bit codewords and the merge bits. In the example shown, the 1 bit included at the beginning of the merge bits causes a majority of the next 17 bits to be ones, thus partially balancing the majority of zeros included in the first 14 bits.
Multilevel optical recording systems have been developed that increase the capacity and speed of optical data storage systems. The term multilevel refers to more than two levels. That is, multilevel optical recording systems include more than two possible levels or states for each location on the optical disc. The density of data recorded on an optical recording medium is increased over traditional optical recording systems that use binary recording by modulating the reflectivity of the optical recording medium among more than two states. Multilevel optical data storage systems, like binary systems, require DC control of the recorded signal to prevent low-frequency content from interfering with the servomotor controls.
The method of introducing or not introducing a single transition using the merge bits does not extend to multilevel signals that are modulated among more than two states. Multilevel channel symbols cause transitions of different magnitudes, or no transition for the channel symbol xe2x80x9czero.xe2x80x9d Moreover, some multilevel systems do not differentially encode the data, and channel symbols are directly mapped to the reflectivity level of the optical recording medium. What is needed is a scheme for providing DC control for multilevel systems.
A system and method for providing DC control for a multilevel system is disclosed. DC control is implemented for systems where input data is either differentially encoded or not differentially encoded. For a differential encoding system, DCC is implemented by selecting and inserting a merge symbol between blocks of data symbols before differential encoding. The merge symbol is chosen so that the resulting differentially encoded signal is DC-balanced. For a non-differential encoding system, DCC is implemented by selectively inverting blocks of data so that the resulting signal is DC-balanced. Merge symbols are selected and inserted between blocks of data symbols. The merge symbols specify whether or not the following block of data is inverted. On the read side, the merge symbols are decoded to determine which blocks of symbols were inverted so that the blocks may be restored to their original state when necessary.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. Several inventive embodiments of the present invention are described below.
In one embodiment, a method of writing a multilevel data sequence to a storage medium so that a read signal generated by reading the multilevel data sequence from the storage medium will have reduced low frequency content is described. The method includes evaluating the effect of a plurality of candidate merge symbols on an RDS of the read signal. A preferred merge symbol is selected from among the plurality of candidate merge symbols based on the effect of the preferred merge symbol on an RDS of the read signal. The preferred merge symbol is added to the multilevel data sequence so that the RDS of the read signal is controlled.
In one embodiment, A DC control encoder in a system for writing a multilevel data sequence to a storage medium includes a processor configured to evaluate the effect of a plurality of candidate merge symbols on an RDS of the read signal and to select a preferred merge symbol from among the plurality of candidate merge symbols based on the effect of the preferred merge symbol on an RDS of a read signal. A bit merger is configured to add the preferred merge symbol to the multilevel data sequence so that the RDS of the read signal is controlled. The resulting read signal generated by reading the multilevel data sequence from the storage medium has reduced low frequency content.
In one embodiment, a storage medium has multilevel data written to the storage medium so that a read signal generated by reading the multilevel data sequence from the storage medium has a reduced low frequency content. The data sequence is written by evaluating the effect of a plurality of candidate merge symbols on an RDS of the read signal and selecting a preferred merge symbol from among the plurality of candidate merge symbols based on the effect of the preferred merge symbol on an RDS of the read signal. The preferred merge symbol is added to the multilevel data sequence so that the RDS of the read signal is controlled.
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example the principles of the invention.